PHYSICIANS often feel impelled to initiate and continue the complete range of modern medical procedures to prolong the lives of critically ill patients. A tragic consequence is that patients who lack decision-making capacity are sometimes given life-sustaining treatment that is not consistent with the patients' previous preferences, not in the patients' best interests, and not wanted by their families.

Why are such cases sometimes handled this way? The reasons are multiple. First, cases involving adult patients who lack decision-making capacity often have medical and nonmedical uncertainties not commonly present in other cases. Some of the uncertainty concerns the questionable decision-making status of patients who have a diminished or fluctuating capacity to make health care decisions because of shock, mental illness, clinical depression, or organic brain syndrome. Other problems arise in cases with patients who have a substantial incapacity to make decisions because of mental retardation, brain damage, severe dementia, or